Diagenesis and copper mineralization in carbonates in the Spokane Formation, Belt Supergroup, at Wolf Creek, Montana

Abstract

Strata-bound silver-bearing copper sulfides occur in two thin green siltstone units, a thin phosphatic green siltstone bed and a stromatolitic oolitic carbonate bed, of the Spokane Formation near Wolf Creek, Montana. Red mudstone and fine-grained sandstone sequences containing sedimentary structures indicative of shallow-water deposition with periodic emergence surround these mineralized zones. The 10- to 30-cm-thick carbonate bed, the lowest and most intensely mineralized unit, crops out discontinuously over 7.5 km. It varies from an ooid-peloid packstone to an ooid-peloid-bearing calcareous siltstone. Chalcocite family minerals, covellite and bornite + or - chalcopyrite, pyrite, and galena occur in the eastern part of the area. Chalcopyrite and pyrite + or - chalcocite family minerals, covellite, and galena occur in the western part.Sulfur isotope values of the sulfides are compatible with bacterial production from seawater sulfate. Oxygen and carbon isotope values of the oolitic limestone are slightly lighter than those obtained for 1 b.y.-old marine limestone elsewhere but are within the range of Ravalli Group and Middle Belt carbonate rocks of the Belt Supergroup.Element analyses show that Cu, Ag, Mo, Pb, and Cd in the mineralized units are enriched over average country-rock values from the enclosing section. Ni and Fe are more abundant in the country rock and Co values are similar in both rock types. Elemental values of the wall-rock samples are very similar regardless of stratigraphic position, color, or grain size.Textures in the oolitic limestone indicate mineralization occurred during diagenesis. Following deposition of calcareous clastics, dolomite, gypsum, and halite formed in evaporitic early diagenesis. Pseudomorphic replacement of these minerals by quartz and albite followed. Pyrite formed prior to silicification and was partially to totally replaced by copper sulfides and galena as waters containing copper and lead apparently moved upward through the sequence. Radial calcite and microspar formed both during and after mineralization, Finally, greenschist facies metamorphism altered the rocks, causing partial to almost complete destruction of the original fabrics and recrystallization of the sulfides in some areas.